Problem: Luis has taken 5 quizzes and his average score so far is 84. If he gets 100, a perfect score, on the remaining 3 quizzes, what will his new average be?
Explanation: If he gets 100 on the remaining quizzes, the sum of his scores will be $5 \cdot 84 + 3 \cdot 100 = 720$ His overall average will then be $720 \div 8 = 90$.